This invention relates to a method and apparatus for monitoring the effective load carried by a crane having a luffing boom of variable effective length.
Any crane has a maximum load rating which cannot safely be exceeded without instability, damage to the crane or even catastrophic failure. The effective load (or load moment) of the crane depends on the magnitude of the load and the effective radius between the point at which the load is attached to the boom and the boom pivot, measured horizontally.
In the case of a crane with a luffing boom, that is, a boom which can be pivoted through a vertical arc, the effective boom radius will be a function of the boom angle. Clearly, the load moment in a crane with a boom of variable length which is carrying a certain load at the end of a fully extended boom which is parallel to the ground will be greater than the load in the same crane, carrying the same load, when the boom is retracted and/or luffed above the horizontal. Thus, a load which may safely be carried by the crane under certain circumstances may exceed the maximum permissible load moment of the crane under other circumstances.